AL-6X stainless steel wire has been chosen as the prime candidate for use as armor wire in the Ocean Therman Energy Conversion (OTEC) power cable. This material contains relatively large amounts of chromium and molybdenus which impart very good pitting and crevice corrosion resistance. These characteristics are of interest because it is anticipated that the OTEC power cable will undergo wave-induced cyclic loading which may cause armor wire corrosion fatigue damage and eventual failure of the cable system. It is, therefore, quite important to carefully document the fatigue characteristics of the Al-6X wire material under simulated service conditions.
Load-controlled, axial fatigue tests have been conducted on Al-6X wire samples in room temperature natural seawater. The initial test show that this material fails transgranularly in fatigue under free corrosion conditions in natural seawater. The material does not show any frequency dependence in fatigue cycles to failure under the conditions investigated. The surface roughness of the wire seems to play an important role in crack initiation. This factor is especially significant in view of the extremely long lengths of armor wire which will be required.
One problem which was encountered during axial fatigue testing of the armor wires was wire breakage in the grips. A new desing of grips was developed which considerably reduced the problem.
Several periodic overstrain fatigue tests were also conducted and they showed the potential for a substantial reduction in the constant amplitude fatigue limit for the material. Under actual variable amplitude loading conditions, it is evident that more realistic life estimates would be obtained if periodic overstrain data were used.
The Ocean Thermal Energy Conversion (OTEC) System is a system which is under construction to generate electricity through the exploitation of thermal gradients between the warmer water at the surface of the ocean and the cooler water several thousand feet below the surface. One of the critical elements in the OTEC System is the electromechanical cable, which is needed to transmit generated electricity to the shore. For this purpose, it is anticipated that a transmission cable will be suspended from a surface vessel or platform to the ocean's floor and it will be hooked up to an electrical cable laid on the ocean floor. The suspended cable, also known as the riser cable, will be strengthened with external armor wires to withstand the wave induced cyclic loading. A major concern has been that these cyclic loads, superimposed on a rather high static load (due to the cable's own weight) may cause corrosion fatigue damage to the armor wires.
Al-6X stainless steel wire has been chosen as the prime candidate for use as the armor wire in the OTEC power cable design. In the next major section of this paper, three factors which will significantly influence the Al-6X armor wire service life are reviewed. Next, a preliminary experimental study on the AL-6Xwire is described. The purpose of the study was to investigate the fatigue strength of this material in room temperature natural seawater.
